1. Field of the Invention
This invention relates to methods for depositing metallic lines on a substrate such as an IC chip or mask, and more particularly to deposition methods employing a focused ion beam to initiate the deposition.
2. Description of the Related Art
Several methods have been employed for depositing lines of material on a substrate in connection with the manufacture or repair of IC chips and masks. These methods include the use of electron beams, laser beams and ion beams. However, none of these methods have been able to achieve a combination of rapid writing, relatively thick lines, resolution of less than a half micron, a high purity of the deposited material, and process compatibility with other focused ion beams (FIB) process techniques.
Prior e-beam approaches have employed both direct polymerization, and chemical vapor deposition (CVD) techniques. Direct polymerization is accomplished at room temperature, with the e-beam left on constantly at a dosage on the order of 10.sup.17 -10.sup.18 electrons/cm.sup.2. The CVD approach is employed at a temperature slightly below the spontaneous thermal decomposition temperature of the gas from which the growth material is extracted. The e-beam is turned off after scanning a line on the substrate, and deposition takes place in the presence of a gas containing the material to be deposited. Although a somewhat lower e-beam dosage is required with the CVD approach, the dosage must still be on the order of 10.sup.16 -10.sup.17 electrons/ cm.sup.2 for reasonable selectivity, representing a dosage reduction of only about 1 order of magnitude. The above temperature and doses have been employed to deposit iron from iron pentacarbonyl gas, but a comparable difference between the direct polymerization and CVD approaches has also been experienced for other materials. Although metal lines with widths in the submicron range have been deposited with electron beams, they typically require high electron doses. The use of electron beams for the CVD of iron lines in the presence of iron pentacarbonyl gas is discussed in Kunz et al., "Catalytic Growth Rate Enhancement of Electron Beam Deposited Iron Films", Appl. Phys. Lett., Vol. 50, No. 15, Apr. 13, 1987, pages 962-64.
Laser assisted deposition has also been employed at temperatures below the spontaneous thermal decomposition temperature of the source gas for the material being deposited. However, in the case of laser assisted deposition the laser spot cannot be focused to a diameter below about 0.5-1 micron. This precludes the use of laser assisted deposition for IC applications having submicron geometries. Laser assisted deposition is discussed in Higashi et al., "Summary Abstract: Nucleation Considerations in the Wavelength-Dependent Activation Selectivity of Aluminum Chemical-Vapor Deposition", J. Vac. Sci. Technol., Vol. B5, No. 5, Sept./Oct. 1987, pages 1441-43.
Focused ion beams have previously been used to deposit metal lines at room temperature in a direct polymerization process. A FIB scans the substrate along the locus of the desired growth line, in the presence of a gas containing the material to be deposited. Unlike the CVD process that has been employed with e-beams and lasers, in which the beams are turned off once nucleation sites have been established and the majority of metal growth occurs by deposition thereafter, the FIB technique has required that the beam be left on during the entire growth period. The previous FIB processes also have several other significant drawbacks. Relatively high doses, on the order of 10.sup.17 ions/cm.sup.2 or greater, have been required to deposit lines that were only a few thousand angstroms thick. The impurity level of the deposited lines is quite high, exceeding 25%, and thereby resulting in a higher resistance than desired. Care also must be taken to ensure that the deposited material is not sputtered away by excessive energy in the beam. The requirement of a relatively high ion dose significantly slows down the speed at which lines can be written, thereby making it difficult to integrate the writing of metal lines with other higher speed FIB processes used in the fabrication of ICs. Thus, although ion beams can easily be focused to below a micron in diameter and thus have the potential for submicron resolution in IC fabrication, a practical way to integrate rapid FIB metal line deposition into an IC fabrication process has not previously been found. The use of a FIB for metal deposition by direct polymerization is described in Shedd et al., "Focused Ion Beam Induced Deposition of Gold", Appl. Phys. Lett., Vol. 49, No. 23, Dec. 8, 1986, pages 1584-86.
Other FIB processes used in IC fabrication are implantation (for selective doping), lithography (for defining small patterns), and sputtering (for removing material). Implantation is generally performed at a dosage within the range of 10.sup.12 -10.sup.15 ions/cm.sup.2, lithography within the range of 10.sup.13 -10.sup.15 ions/cm.sup.2, and sputtering at about 10.sup.16 -10.sup.17 ions/cm.sup.2. A FIB metal deposition technique that required ion dosages comparable to these other processes would make possible the totally mask-free fabrication of ICs. Unfortunately, such a technique has not previously been available.